The pistachio tree, Pistacia vera, is a deciduous tree noted for its edible nuts. Native to arid regions of Asia and Asia Minor, pistachio trees are grown primarily in the Mediterranean region, Italy, Turkey, Iran, Greece and The Soviet Union as a nut crop.
The green or reddish oval fruits, generally 3/4 to 1 inch in length, are borne in clusters (racemes) on previous years vegetative growth of the pistachio tree. The fruit consists of an outer, fleshy hull (epicarp) containing a thin, tough shell. This hard shell contains the usually light-green nut kernel. The pistachio hull usually splits at maturity and is easily separated from the shell at harvest time.
The shell encasing the pistachio nut is divided by a longitudinal seam consisting of living cells. This seam splits, prior to nut harvest, in a fraction of the nut crop, resulting in a split-shell pistachio nut.
In harvesting pistachio nuts, the tree is shaken mechanically, to remove the nuts. The nuts are hulled immediately and dried.
The hulled nuts are separated from the blanks by an airleg or water bath flotation before drying and then sorted between split-shell pistachio nuts from intact-shell nuts ("non-splits"). The split-shell nuts are then generally roasted and salted, and typically sold as a popular snack food. As split-shells ease shelling by the consumer, these nuts command a higher price. The non-split nuts are cracked mechanically and the nut kernels are sold to the baking, ice cream and confectionery trade.
Although pistachio nuts are grown in California and other parts of the arid Southwest, a substantial commercial industry has been slow to develop in the United States, due in part to the long delay between initial tree planting and nut production.
Pistachio trees, producing both non-splits and split-shell nuts, are severely alternate bearing. In the "off years," non-splits account for 10 to 25% of the crop and the problem substantially worsens in the "on years." This imposes severe financial burdens on pistachio nut growers, since the market value of the non-splits is far below that of the split variety. For example, in 1986 approximately 55 million pounds of split-shell nuts and 20 million pounds of non-splits were produced from 40,000 acres, the non-splits representing over 38% of the average grower's crop (California Pistachio Commission Annual Report 1986/87). These nuts sold for $0.25 per pound before cracking, while the split-shell nuts commanded six times this amount, or $1.60 per pound. The growers' lost income amounted to approximately $25 million or over $600 per acre.
A resolution to the non-split problem has eluded both growers and university researchers. Experimenters have varied such parameters as fertilization, harvest timing, rootstock, nut variety and cultural practices, all to no avail. It would therefore be desireable to utilize a method to promote shell splitting in pistachio nuts prior to harvest. It would also be desireable to employ a composition which would promote shell splitting in pistachio nuts prior to harvest.